Classical Thomas peaks in various fast second-order particle transfer processes are quantum mechanically broadened by energy nonconservation in the intermediate states of a collision. This quantum broadening is considered in observable velocity distributions of recoil particles.

It is shown that permanent, solitonlike structures in which the pump retains a large fraction of its initial energy cannot exist for finite energy pulses. Ultimately, solitonlike structures propagate to the back end of the pulse and disappear. A method for propagating pump energy over long distances is proposed.

It is demonstrated in a photon coincidence experiment with two photodetectors, in which signal and idler photons produced by parametric down-conversion are allowed to interfere, that the visibility of the interference pattern is well above 50% and remains unchanged when one of the two light beams is attenuated ninefold compared with the other. These results violate classical probability for light waves.

We observe that the specular reflectivity from the input face of a ${\mathrm{BaTiO}}_3$ phase-conjugate mirror decreases by over 600% relative to the standard Fresnel reflectivity value upon the onset of phase conjugation, or wave-front reversal. Reasonable agreement is obtained using a model involving the destructive interference of the Fresnel-reflected beam with a series of phase-conjugate wave generated internal to the crystal. The basic diminishing effect should be universal and hence observable in other classes of self-pumped and externally pumped phase conjugators.

We report the first direct observation of the electromagnetic deflection of high-energy electron and positron beams as they pass each other with small impact parameters. Measurements of the deflection amplitude are found in agreement with theoretical expectations. This phenomenon, which is sensitive both to the relative position of the two beams and to their transverse sizes, has been used successfully to optimize and maintain collisions at the interaction point of the SLAC Linear Collider.

Fermat’s principle shows that a definite convex set of feasible slowness models, depending only on the traveltime data, exists for the fully nonlinear traveltime inversion problem. In a new iterative reconstruction algorithm, the minimum number of nonfeasible ray paths is used as a figure of merit to determine the optimum size of the model correction at each step. The numerical results show that the new algorithm is robust, stable, and produces very good reconstructions even for high contrast materials where standard methods tend to diverge.

In this Letter we study subcritical transitions in spatially extended systems when topological defects are present. The behavior of a single defect is discussed in the framework of Ginzburg-Landau models. We show how the transition from one stable state to the other is affected by the presence of defects. Our results, claimed to be independent of these models, are illustrated with examples coming from Couette-Taylor, liquid crystals and convective instabilities.

We have directly measured the time-dependent electromagnetic shock wave generated by an electric surface-dipole distribution propagating faster than the phase velocity in the underlying dielectric substrate. The results are in good agreement with a theory describing the moving volume-dipole distribution with the inclusion of an equal strength surface reflection.

The question of whether it is possible to have a kinematic magnetic dynamo for a conducting fluid with time-independent (steady) velocity field v(x) and vanishingly small electrical resistivity has remained an open question in the understanding of the origin of magnetic fields in nature. By considering the zero resistivity dynamics, examples of steady dynamos are found. Analysis of these examples supports the conjecture that, for sufficiently small resistivity, 2 dynamo action can occur in typical, smooth, steady, three-dimensional, chaotic fluid flows.

A spirally deformed surface due to a single-atomic shear of a screw dislocation emerging on a GaAs(110) surface has, for the first time, been observed in the precision of ∼0.01 Å by interferometrically measuring the phase of the reflected electrons. In the case of an obliquely emerging dislocation, the surface around the dislocation core was observed to be deformed rather asymmetrically than uniformly. The result can be explained by considering surface stress relaxation.

A mechanism linking high-frequency microinstabilities and the low-frequency macrostability is proposed. The coupling is provided by the time-averaged force, ponderomotive force, of unstable high-frequency waves. Two specific examples of this phenomenon are discussed. It is shown that an α-particle loss-cone instability stabilizes the flute mode of an ignited, axisymmetric mirror plasma. In tokamaks, the ion-whistler instability, driven by an anisotropic population of energetic particles, stabilizes the internal kink mode for JET range of parameters.

Analytic arguments are presented, concerning the ‘‘phase transition’’ to nonmultifractal behavior of the qth moment, $M_q$, of growth probabilities in diffusion-limited aggregation, found numerically by Lee and Stanley. Assuming the existence of exponentially small growth probabilities, for a single growing aggregate, we find a transition at q=0. For aggregates of size L, this transition splits into two at $q_0$(L)<$q_c$(L)<0. Quantitative analysis of $q_0$(L) yields information on the tail of the growth probability distribution. Averaging $M_q$ over all aggregates may yield a finite $q_0$.

We present a comprehensive theory of fluids which has the typical accuracy of a good liquid-state theory in the dense regime but in addition has a genuine nonclassical critical behavior. The theory is based on the hierarchical reference theory of fluids decoupled with an approximation inspired by the optimized random-phase approximation. The Lennard-Jones interaction is studied in detail above the critical temperature.

Charge-density-wave response from depinning at low fields and frequencies can be determined from the response in the neighborhood of the pinning frequencies. An abrupt depinning observed at very low temperatures occurs at a threshold field such that the polarization energy becomes larger than the net pinning energy. For both types of depinning, there is a universal threshold polarization per conducting chain, independent of material parameters, but smaller for the abrupt threshold, in agreement with experiment.

The problem of simultaneous diffusion and recombination of electron-hole pairs, photoexcited in noncrystalline semiconductors at low temperatures, is reduced to a universal mathematical problem whose solution does not depend on the density-of-states function. We derive a general geminate-recombination function which at low light intensities describes the distribution of radiative recombination times. The low-temperature photoconductivity is found to depend only weakly on material parameters and agrees with experiments on hydrogenated amorphous silicon.

Quantum mechanical calculations of electron scattering at junctions of 1D conductors are presented together with an analysis of the implications for experimentally measured transport coefficients. Resonant electron states intrinsic to such junctions give rise at low B to quenching of the Hall voltage, maxima in the longitudinal resistance, and anomalous Hall plateaus. At high B they show up as sharp features in $R_H$ and $R_L$ which track the bottoms of 1D subbands. Quenching of the Hall voltabe is found for ≤3 populated 1D subbands. The other resonant phenomena occur also at higher band fillings.

We report the first observation of strong intersubband (e1-e2) absorption by photogenerated excitons in GaAs/GaAlAs undoped, multi-quantum-well structures. The origin of the transition is verified by polarization measurements, temperature dependence, and comparison with photoluminescence data. Its oscillator strength is found to be approximately 25 times stronger than that measured for bare electrons in modulation-doped quantum wells. We attribute it to an excitonic enhancement.

If two anisotropic superconductors are in perfect contact, the persistent current normal to a sufficiently long interface must be accompanied by spontaneous creation of a vortex chain at the boundary (in zero external field). The phenomenon is due to peculiar boundary conditions which should be imposed upon currents at the interface. The vortices are subject to the Lorentz force along the interface, thus providing the possibility of an extra dissipation and reduction of the critical current across the grain boundary.

The quantum dynamics of the Landau-Zener transition in a dissipative environment is studied and analytical results for the transition probability are given in terms of temperature, coupling strength, and the Landau-Zener time. For short Landau-Zener time there is no effect of dissipation. In the oppotise limit we distinguish various temperature regimes: The adiabatic limit is shown to be restricted to low temperatures and no effect of dissipation is present at zero temperature. At intermediate temperatures thermal transitions dominate for weak coupling and high temperatures correspond to the strong-coupling limit.

We study the spectral statistics of systems of two-dimensional pseudointegrable billiards. These systems are classically nonergodic, but nonseparable. It is found that such systems possess quantum spectra which are closely simulated by the Gaussian orthogonal ensemble. We discuss the implications of these results on the conjectured relation between classical chaos and quantum level statistics. We emphasize the importance of the semiclassical nature of any such relation.